Capillary electrophoresis is known for the advantages it offers in certain applications. One advantage is that it permits use of high voltages by virtue of the ease of cooling a capillary tube. High voltages are particularly useful in performing electrophoretic separations at high speed. In addition, the small diameter of the capillary is well adapted to the separation of extremely small samples, making the system particularly useful with samples which are obtainable only in small amounts. Capillary electrophoresis also lends itself to the use of a buffer solution as the separation medium, avoiding the use of complex media such as gel or paper yet avoiding band broadening. Still further, capillaries by virtue of their flexibility can be used to provide a relatively long separation path permitting the separation of a large number of components in a single sample, including those which are closely related. Further still, capillary tubes are well suited to on-line detection of the separated species by the use of a light beam passing through the capillary at a point toward the exit end, the emerging beam directed to a detector.
Disclosed herein are capillary electrophoresis systems with further useful features, broadening their range of application and further facilitating their use. In particular, this invention provides the repeated replacement of a segment along the solute migration path. In accordance with this innovation, a series of volumes of liquid medium may be placed successively in the path of the electric current, each one subsequently removed for replacement by a fresh volume. The process may be repeated in an extended sequence, or as few times as once. The invention lends itself to automation, as well as a high degree of precision in terms of volumes, migration path lengths and contact time intervals.
The invention has two primary embodiments, one in which the interchangeable segments are positioned at the downstream end of the larger, main body of the capillary, and the other in which the segments are placed at the upstream end. In the former, the segments are used to separate the species eluting from the larger capillary into separate receptacles, from which they may be separately recovered, detected, treated, or otherwise processed. In the latter, the segments may be used to load a series of samples onto the larger capillary in sequence at controlled time intervals. In further embodiments of the invention, the two features are combined, thereby achieving both repetitive and sequential sample loading at one end of the capillary and the separation and isolation of eluant fractions at the other.
In all embodiments, the interchangeable path segments are in the form of capillary passages of relatively short length compared to the length of the capillary through which the main portion, if not the entirety, of the electrophoretic separation takes place. The short interchangeable passages will be referred to herein for convenience as "capillary segments," whereas the longer length of capillary in which the major portion of the separation takes place and which remains in the current path at all times will be referred to as the "separation capillary," This invention extends to all systems in which the capillary segments and separation capillary move with respect to each other, thereby bringing the capillary segments one at a time into fluid communication (and electrical communication) with the separation capillary. In preferred and most convenient embodiments, the separation capillary will be mounted or supported in a stationary manner and the capillary segments will be mobile, their motion controlled in accordance with their intended function. The separation capillary will most conveniently be retained and supported in a housing or support, such as for example an enclosed cartridge, such that the capillary end which communicates with the capillary segments is readily accessible to them and can be readily aligned and placed in fluid contact with the segments in succession. The capillary segments are conveniently contained in a common block or structural member which can be drawn across the exposed opening of the separation capillary. In certain embodiments, a fluid-tight seal is maintained at the juncture between the capillary segments and the separation capillary, and in further embodiments, this seal is one which will prevent leakage when the system is placed under a positive pressure such as the pressures encountered during pressurized sample loading.
Further features and advantages of the invention will be apparent from the description which follows.